Conventional wood-based composite products are generally made using thermosetting or heat-curing resin or adhesive to bind the lignocellulosic material (wood substrate) together. The resin-binder systems generally include phenol-formaldehyde, urea-formaldehyde, melamine-formaldehyde and isocyanate. Phenol-formaldehyde (PF) resins are generally used in the manufacture of products that require durability with respect to exterior exposure, such as plywood, OSB and siding. Such resins require a longer press time and higher press temperature than do products made with urea-formaldehyde resins. Products made using phenol-formaldehyde resin thus have a slower thermal cure, which may be lengthened due to the need to eliminate moisture from the wood substrate during curing. It is known that the curing time may be shortened by the use of additives such as organic or inorganic acids or hexamethylenetetramine, but the additives may cause side effects that are undesirable. The longer press time and higher temperature needed for proper curing result in higher energy consumption and lower line speeds, causing a lower production rate. When phenol-formaldehyde resins are used to make products, the products may have a lowered dimensional stability as a result of a lower moisture content in the finished product. Also, since products manufactured with phenol-formaldehyde resin may tend to be dark in color, such products may be unsuitable for decorative applications such as furniture.
Plywood is a flat panel containing sheets of veneer called plies. Thus, plies are individual sheets of veneer in a panel. The plies are bonded using heat, pressure and a bonding agent to create a panel with an adhesive bond between the plies. Plywood may be made from hardwood, softwood, or a combination of both. Generally, plywood is constructed with an odd number of layers, with each layer having a grain perpendicular to the grain of the previous layer. Layers may be a single ply or a plurality of plies that are laminated such that their grain is parallel. The inner plies are called cores or centers. The outside plies are generally called faces. The inner plies may vary as to number, thickness, species and grade of wood and generally have a panel thickness of 8 mm to 45 mm. Thus, a panel may have an odd or even number of plies, but normally will always have an odd number of layers, i.e., the number of layers is determined by the number of times the grain orientation changes, and the outer layers generally have their grain direction oriented parallel to the length of a panel, while the inner layers generally have their grain direction oriented perpendicular to the length of the panel. Hence, a panel may be described as, for example, four ply, three layer. Alternating grain directions for adjacent plies provides dimensional stability across the width and axial strength in a direction perpendicular to the panel plane. Lamination provides for distribution of defects and minimizes splitting propensity.
Two classes of plywood are generally produced: construction and industrial, and hardwood and decorative. The construction and industrial plywood may include hardwood, and is classified by exposure capability and grade, where exposure capability may be exterior or interior. Exterior capability plywood may not have less than “C” grade veneer, as determined in Product Standard 1, an industrial standard known to those skilled in the art. Interior capability plywood may be “D” grade veneer. Hardwood and decorative plywood may include certain decorative softwood species for non-construction use, but generally includes four types of plywood, listed here in decreasing order of resistance to water: Technical (Exterior), Type I (Exterior), Type II (Interior), and Type III (Interior). The adhesives used in the manufacture of the two classes of plywood are generally different, but are selected to achieve the desired specifications of the end-product wood composite.
After the trees are felled and cut into logs, the logs are graded and sorted to provide the most efficient use of the timber. Lower grade logs may be graded as “peelers”, and higher grade logs may be graded as “sawlogs”. High grade peelers and high grade sawlogs are sent to sawmills. The lower grade peelers and lower grade sawlogs are sent to the veneer mills to make plywood. At the veneer mill, the logs are sorted by grade and species, then debarked and cross cut into peeler blocks. The peeler blocks may be heated, steamed, or immersed in hot water prior to peeling to facilitate peeling and provide better quality veneer material. The peeler blocks are then transferred to a veneer lathe where the blocks are rotated at high speed and are fed against a stationary knife parallel to the length of the block until a predetermined size core remains. The core may be sawed into lumber, sold as fenceposts or timber or chipped to provide wood chips. Thus, veneer is peeled from the block as a continuous, uniform, thin sheet. Veneer typically is 0.8 mm to 4.8 mm thick.
The continuous sheet of veneer is then cut into usable widths; defects are removed; and the veneer may be dried to a moisture content that is compatible with the adhesive being used to bind the panels. Glue may be applied to a panel by any known method such as spraying, curtain coating, roller coating, extrusion, or foaming. Adhesive may applied to one surface by spraying, curtain coating or foaming, or may be applied to two surfaces by using a roll-coater. Typically, after being coated with adhesive, the core plies are “laid up” to form desired panels and then conveyed from the lay-up area to the pressing area. Generally, the panels are cold pressed to flatten the veneers and transfer the adhesive to uncoated sheets, followed by hot pressing to cure the adhesive. Then panels are solid-piled (hot-stacked) to allow the adhesive to complete cure. Then panels may be sawed to a desired size, sanded where desired, and then graded.
Generally, in part due to their light color, urea-formaldehyde (UF) resins are used for manufacturing interior or decorative products. Products prepared using urea-formaldehyde resins tend to have a smooth surface and uniform dimensions. Curing for such products is generally accomplished at a temperature similar to PF plywood resins, but lower than PF OSB resins, and press times are generally shorter. Due to the cost of raw materials and processing, urea-formaldehyde resins are more economical than phenol-formaldehyde resins to use for manufacturing composite wood products.
Since melamine-formaldehyde (MUF) resins are generally more expensive than phenol-formaldehyde resins, melamine-formaldehyde resins are typically blended with urea-formaldehyde resins and utilized for wood composites used for decorative applications.
Diphenylmethane di-isocyanate often is used to manufacture composite wood products such as OSB. However, the highly toxic nature of the isocyanate requires that special safety precautionary measures be implemented in the manufacturing process.
Clearly, selection of adhesives in the manufacture of composite wood products requires consideration of a number of factors such as, for example, the total costs to be incurred, the materials to be bonded, the moisture content at the time of bonding, and the desired properties and durability of the products to be manufactured. Generally, phenol-formaldehyde and urea-formaldehyde are the most commonly used adhesives for lignocellulosic, i.e., wood-fiber, composite products.
A number of products contain particle and fiber composite materials. OSB and waferboard are made by flaking or chipping roundwood. Fiberboard is made by reducing chips to wood fiber wherein steam is usually used to soften the wood. After softening, the wood is partially dried, adhesive is added, and a mat of strands, particles, and fibers is produced. A platen-type press is used to press the mat using heat and pressure to cure the adhesive. The bonded mat is then cooled and cut into desired sizes.
Fiberboard includes hardboard, medium density fiberboard (MDF) and insulation board. Fiberboard may be manufactured by attrition milling, in which the wood is fed between a rotating disk and a stationary disk. As the wood is fed between the disks, it is forced into fibers and groups of fibers. This process may be enhanced by, prior to attrition milling, soaking the wood in water, steaming the wood, or chemically treating the wood. Fiberboard, may be made by a wet or dry process, but, due to having longer fibers of wood than particleboard, tends to be a stronger product than particleboard. In the wet process, the fibers are mixed with water to form a pulp, which is then placed on a continuously moving screen or on cylinder formers. This process removes some of the water, and the resulting mat is put through press rolls to remove excess water. Steam-heated presses are used to further process the fiberboard. The fiberboard may then be dry heated to improve the resin bonding. Also, oil may be added to provide protection from moisture. Alternatively, the fiberboard may be continuously or progressively humidified to increase the moisture content from about zero after pressing, to about three to seven percent moisture.
Waferboard was originally manufactured from aspen wood strands that were bonded together with resin using heat and pressure. As demand increased, waferboard production evolved into production of a related product, OSB (Oriented StrandBoard). The OSB includes woods other than aspen, including small amounts of certain hardwoods. OSB is manufactured from long, thin wood strands that are bonded together into mats with waterproof resin and in some instances wax, placed in a perpendicular orientation in adjacent layers as described above for plywood, and pressed using heat. Typically, logs are debarked, then may be soaked or sent directly to be processed into strands. Generally, if the wood is green, the strands may be stored in moist bins prior to drying. After drying, the strands are blended with adhesive and wax, wherein the mat face and the core mat each have different resin formulations based on the desired end product. Since OSB is generally configured in layer mats consisting of a mat face, a core mat and another mat face, the mat faces typically have the wood strands oriented parallel to the length of the board, and the core mat has the wood strands oriented perpendicular to the length of the board. The layer mats are hot pressed to cure the resin.
In order to avoid waste of leftover materials such as sawdust and trimmings of wood products, the product particleboard evolved. The leftover materials were first reduced to small particles, adhesive was applied, a mat was formed, and heat and pressure were applied to the mat to provide a panel product. Mats typically have a moisture content of eight percent to twelve percent prior to being pressed and generally have a moisture content of approximately five percent to nine percent after pressing. Particleboard may have a resin content of four percent to ten percent, but particleboard made with UF resins typically has a resin content of six percent to nine percent. For interior use particleboard, UF resin is generally used for the adhesive. However, to provide a moisture-resistant particleboard, PF and MF resins are sometimes used. It should be noted that particleboard may also be made by an extrusion process wherein the particles are forced between two platens of a heated die, and the particleboard is extruded between the platens. Since the particles are typically aligned perpendicularly to the plane of the particleboard, the particleboard made by extrusion generally has properties different from the properties of particleboard made by hot pressing into panels.
Since wood differs widely between species and since moisture uptakes by different species may vary during processing, moisture variations may occur as well as temperature variations, making uniformity, consistency and predictability for a composite product difficult to obtain. Since the substrate temperature and moisture content of the composite product are essential to produce a reliably consistent adhesively bonded product, there is a need for a system for maintaining application of a catalyzed thermoset adhesive using a feedback process that permits variation of the level of catalyst blended in the resin based on the variation of at least one of the temperature and the moisture content of the lignocellulosic material (wood substrate).